There is a driver to that backwards moving car. As an audience, we could see from Kat's point of view that:
- when Kat and Sator are about to enter the warehouse where there is a time portal, Kat notices on the other side of the fence, an Audi car parks there and its driver has an oxygen mask.
There are at least four cars involved: the Audi, the BMW, the crashed car, a second car. From an audience point of view, we initially see:
- The Audi has a driver wears an oxygen mask, parked outside a large warehouse.
- The BMW driven by Neil and has the protagonist in the passenger seat has a broken mirror.
- Neil and the protagonist notice the Audi is moving backwards towards them, scratch their broken mirror and start chasing them. In the Audi, we see Sator, Kat and a driver (at the edge of the screen or even completely off the screen)
- An oxygen wearing mask Sator asks the protagonist to handle over the device.
- A crashed car in the middle of the road start moving backwards in time and manages get in between the Audi and the BMW.
- Sator gets hold of the device and a second car comes over next to the Audi, everyone except for Kat jumps onto the second car from the Audi quickly (this is shown very briefly)
- Before the driver and Sator jump off their Audi, they must have somehow make the Audi to drive in a cruise mode. Therefore the Audi is moving in a straight line (and almost crashes into other cars should the protagonist not stop it in time) and no driver is there to steer it.
There is so much going on screen, it is easy to not see them leaving the Audi.
The Audi car is not really moving backwards by the way, it is being driven forwards because it is inverted. From the Audi point of view, most cars (apart from the crashed car and Sator's own second car) are moving backwards in time therefore it is more intuitive for that inverted Audi driver that car that way.